1. Field of the Invention
The present invention relates to a copier, facsimile apparatus, printer or similar image forming apparatus and more particularly to a fixing device included in an image forming apparatus for heating a toner image carried on a sheet or recording medium.
2. Description of the Background Art
Generally, an image forming apparatus includes a fixing device configured to fix a toner image transferred to a sheet or recording medium with heat and pressure. A heat roller type of fixing system belongs to a family of conventional fixing devices and includes a heat roller and a press roller facing each other. The heat roller and press roller convey a sheet carrying a toner image therebetween to thereby fix the toner image with heat and pressure.
In the heat roller type of fixing system, a heat source may be disposed in the heat roller, as taught in, e.g., Japanese Patent No. 3,153,732 (column “0015, FIG. 1). Alternatively, a heat source may be positioned outside of the heat roller for heating the surface of the heat roller, as proposed in, e.g., Japanese Patent Laid-Open Publication Nos. 8-129313 (columns “0055” and “0056”, FIG. 2) and 10-133505 (column “0110”, FIG. 1).
Further, to promote rapid warm-up of the heat roller heated by the heat source, Japanese Patent Laid-Open Publication No. 11-84933 (column “0022”, FIG. 1), for example, discloses a fixing device configured to sense the temperature of the heat roller at positions preceding and following a sheet conveying zone and feed, when a pressure difference reaches a preselected value, power to the heat source. Also, Japanese Patent Laid-Open Publication No. 2002-72731 (column “0049”, FIG. 2), for example, proposes to provide the surface of a heating member with light absorbing capability. Further, Japanese Patent Laid-Open Publication No. 10-10919 (column “0033”, FIG. 1), for example, proposes to feed power to the heat source when the surface temperature of a heating member drops below a preselected value.
However, the conventional fixing devices cannot surely prevent the surface temperature of the heating member from dropping, particularly in the sheet conveying zone or nip, extending the warm-up time of the heating member.